A kind of a nonaqueous secondary battery in which a positive electrode is a metal oxide, an electrolyte is an organic electrolytic solution, a negative electrode is a carbon material such as graphite, and a porous separator is used between the positive electrode and the negative electrode is called as a lithium-ion secondary battery (also simply referred to as a “battery”). Since its production at first in 1991, it has been rapidly and widely employed in view of its higher energy density as a battery for a mobile instrument such as a mobile telephone which has been reducing in size and weight.
In addition, a lithium ion secondary battery (large-capacity battery) having enlarged capacity to store power generated has been researched. It was also reported that the large-capacity battery was manufactured by simply upsizing a conventional battery in a case.
Since the lithium-ion secondary battery employs the organic electrolytic solution as the electrolyte, several measures are taken to prevent incidents such as blowout, fire or the like under harsh conditions. One of the measures is to assure safety by a function called “shut-down function”, that is, when the battery temperature rises, the separator melts to bung up holes of the separator, so that the current is shut down.
However, the problems associated with the safety of the battery have arisen even for the batteries to which these measures are taken. For example, a current flow is concentrated in a short-circuited portion due to external obstacles (a nail is stuck or the like) or internal obstacles (mixing extraneous matters or the like) to generate heat by resistance heating, so that a chemical reaction of an active material and the electrolytic solution in the battery is caused by the heat. As a result, the so-called “heat overdrive” is caused in the battery so that an incident such as blowout or fire happens at the worst.
Among a number of suggestions for the measures, there is one suggested lithium-ion secondary battery in which a current collector composed of a resin film and a metal layer on the surface of the resin film is used for the positive electrode and/or the negative electrode (Patent Document 1: Japanese Unexamined Patent Publication No. HEI 11(1999)-102711).
In the battery including such a current collector, when abnormal heat generation arises, the resin film is melted down to break the positive electrode and/or the negative electrode. As a result, the temperature rise in the battery is restricted to prevent fire.